Pickup trucks may include a rear window assembly that defines part of a passenger compartment of the vehicle. The window assembly may include a movable pane that selectively closes an opening to the exterior of the vehicle. The movable pane may be driven by an actuator or manually opened. Some movable panes are flush with adjacent fixed panes when the movable pane closes the opening. These movable panes have a compound motion or path of travel between their open and closed positions that permits the movable pane to move into and out of plane with the fixed panes so that the movable pane can be laterally moved relative to the fixed panes and the opening. It can be difficult to reliably close the movable pane against seals surrounding the opening, and to control compound movement of a flush sliding window.